Abstract Large, rounded idioblasts were observed in adaxial leaves of mulberry plants; they were clearly distinguishable from epidermal, trichome and parenchyma cells. The size and density of idioblasts varied according to leaf age. Cytological features of idioblasts were as follows: the outermost region (‘cap’) of idioblasts was situated on the adaxial surface as a dome-like protrusion; a cylindrical protuberance extended from the cap region to the inner part of the idioblast; in idioblasts from mature leaves a crystal mass was suspended from the lower tip of the cylindrical protuberance. Elemental analysis of idioblasts demonstrated that silicon (Si) was localized in both the cap region and the cylindrical protuberance but calcium (Ca) was present in the large crystal, indicating site-specific cellular localization of Ca and Si within an idioblast. Histochemical assays showed that a distinct Ca crystal filled the vacuoles of idioblasts in mature leaves, while immature leaves had many idioblasts without Ca deposition. The increase in the Ca content of leaves was directly proportional to the increase in leaf age and appeared to be closely related to the Ca sink capacity of the developing idioblast vacuoles. The maximum sink capacity was quantified to be approximately 40 ng per idioblast when mulberry plants were grown hydroponically with excess Ca.